The present invention relates to an apparatus and method for controlling an electric motor and, more particularly, to a method and apparatus for controlling an electric assist motor in an electric assist steering system.
Motor control systems for electric assist steering systems typically are equipped with various circuitry and controls for monitoring and detecting abnormal conditions in the electric motor being monitored. For example, it may be desirable to detect when the electric motor is not commutating properly or when the motor or the drive circuitry has overheated. U.S. Pat. No. 5,257,828 and U.S. Pat. No. 5,517,415, both to Miller et al. and assigned to TRW Inc., disclose approaches for detecting abnormal conditions in an electric assist steering system and for controlling the electric assist motor accordingly.
U.S. Pat. No. 5,726,545 discloses a method for controlling electric current in a servo motor using DQ conversion for converting three-phase current into a two-phase DQ reference coordinate.
U.S. Pat. No. 5,670,854 to Matsuura et al. discloses a control system for an induction motor in an electric assist steering systems where the induction motor does not include permanent magnets. The Matsuura et al. patent further discloses using DQ conversion in the control process for the electric assist motor.
The present invention is directed to a method and apparatus for controlling an electric assist steering system. A first set of motor control values are determined in response to applied steering torque. The electric motor is energized in response to this first set of motor control values. When the motor is energized, it has motor operating values. Motor control values are determined from the motor operating values. A second set of motor control values are determined in response to applied steering torque. The motor is disabled when the determined motor control values from the motor operating values is inconsistent with the determined second set of motor control values responsive to applied steering torque.
In accordance with another embodiment, motor current control values are determined in response to applied steering torque. A DQ controller transforms the determined current control values into three-phase drive signals which are applied to the motor. DQ values are determined from the monitored motor current and position values. DQ values are determined in response to applied steering torque. The DQ values responsive to applied steering torque are compared against the DQ values responsive to monitored motor current and position. The motor is disabled if the comparison indicates an inconsistency.